Substance P, an undecapeptide, has been identified by combined immunohistochemical and radioimmunological techniques to be present in nerve fibers and terminals in amphibian, mammalian, and avian autonomic ganglia. The peptide has been shown to depolarize some sympathetic neurons of frog and guinea pig and to modulate the activation of the acetylcholine receptor (AChR) in chromaffin cells and in embryonic chick autonomic neurons. The physiological role of the peptide in ganglionic transmission remains uncertain, however. The long-range objective of this work is to define the precise physiologic effects of substance P and other peptides in the modulation of cholinergic transmission in developing chick autonomic ganglia. The specific aims of this proposal focus on first, a determination of the effect of substance P on the activation of the cholinergic receptor. This includes a detailed electrophysiological characterization of substance P's inhibition of ACh-induced inward currents in embryonic autonomic neurons in vitro. Second, the pharmacology of the effect will be studied to examine the action of substance P on the dose dependence of ACh's interaction with the AChR as well as to characterize the substance P receptor in detail. Third, studies will be initiated to examine the mechanism of substance P action at the cholinergic receptor using voltage clamp recording for fluctuation analysis of receptor kinetics as well as single channel recording. The final aims of this proposal will extend the observation of substance P effects on applied ACh to studies of the peptide at cholinergic synapses in vitro and in vivo. The studies that comprise this proposal constitute the groundwork for the first attempt to describe the development, physiology, pharmacology and biophysical mechanism of substance P modulation of cholinergic synapses in autonomic ganglia. Since the function of the peptide is largely undefined, these studies should yield fundamental and important information about the physiology of substance P. Furthermore, since the peptide is present in central as well as peripheral nerve structures, the implications of this study reach far beyond the ganglia.